Classes of Coloring Material

These retain the planar molecular structure of acid direct dyes, but cationic groups have been incorporated in the structure. This is to increase their affinity to paper fibers. They are moderately adsorbed on bleached Hgnin-containing stocks, and even in the case of deep shades, alum or a fixative is usually not required. These dyes produce fairly good bleed fastness. Combination dyeing with anionic direct dyes can be carried out to improve the fixation of the dyes on the paper machine 

Anionic fixatives, e. g., the sodium salt of condensation products of formaldehyde and naphthalene sulfonic add, also form color lakes of this type with cationic dyes. They are used to improve the fixation of basic dyes on bleached pulps that contain only small amounts of hgnin. Better dye fixation always results in richer colors, improved bleed fastness, and less effluent staining. 

These are all water-soluble salts (usually in the form of the sodium or potassium salt) of colored organic acids which dissociate in water to form colored anions. Most acid dyes are azo dyes and are similar to direct dyes. These two groups overlap with no distinct boundary. Add dyes generally contain more add groups which gives rise to their greater solubihty in water, compared to direct dyes. 

Colored pigments are applied more often in surface coloring and coating formulations where they are superior to soluble dyes, because they do not follow the water into the paper sheet when the starch or size or pigment coating is appHed. 

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Fig. 3.7 Classes of coloring materials in 2004 (market shares at world wide turnover of 500 x lO ).

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